This invention relates to printing cylinders and rollers for printing machines, particularly to offset printing cylinders, impression cylinders, form cylinders and dampening rollers, the surface of which has to be highly corrosion-resistant and also resistant to the influence of chemicals which are used for printing.
The surface of the above-named cylinders and rollers comes into contact with water, printing colors and different chemicals which are used for printing on printing machines. These substances cause a damaging corrosion on the surface of the printing cylinders. In addition to highly corrosion-resistant printing cylinders on printing machines, dampening rollers as well must have a sufficiently water-adhesive surface to insure a proper transfer of dampening liquid on the surface of the printing plate. The surface of said cylinders and rollers is generally finished by grinding to assure a smooth finish for the cylinder surface. An accurate coaxial positioning of the cylinder axis relative to the cylinder surface and a good surface quality are essential for good printing quality.
One of the known cylinders for printing machines is produced from a casting of high quality metal alloy, whereby the surface of the cylinder is accurately ground. A disadvantage of this type of cylinder is that it is very expensive to produce because quality alloys are very expensive. A further disadvantage resides in that it is difficult to obtain a metal alloy with optimum casting properties, particularly since very complex shapes have to be cast as, for example, the ribbed cavities of a printing cylinder.
There are also known printing cylinders for printing machines made from cast metal, the surface of which has to first be ground and then provided with a chromium layer which is about 10 to 15 millimicrons thick. The advantage of such a thin layer of chromium is that the surface of the cylinder does not require any further operation or additional working.
A disadvantage with such a cylinder provided with a layer of chromium is that on the edge portions of the cylinder there form projections of accumulated chromium which result in an inaccurate geometry of the cylinder or roller. Another disadvantage resides in the physical properties of such a thin layer; for example, such a thin layer provides reduced corrosion resistance.
There are known other cylinders of printing machines onto which a relatively thick layer of chromium is applied after surface grinding of the cylinder. However, such cylinders having such a thick layer of chromium must subsequently be ground to obtain a suitable surface. Such a grinding operation increases the production costs thereof considerably. The application of such a thick layer of chromium also requires substantially larger capacity chrome bath vanes.
Rollers for printing machines, for example, dampening rollers or distribution rollers, are made from a tube into which pivots are pressed from the sides, or are turned from solid bar material. Such rollers must be highly corrosion-resistant in order to function properly and to have the ability to transfer dampening liquid. Such rollers are, therefore, generally made from anti-corrosive alloy steel or the surface of such rollers are chromium-plated. One disadvantage of such rollers made from anti-corrosive steel is their high cost. On the other hand, chromium-plated cylinders and rollers are not sufficiently corrosion-resistant.
In addition to the aforementioned corrosion-resistant property, the surface of impression cylinders for multi-color printing machines, or for printing machines for printing on one side of a sheet of paper and, after adjustment, printing on both sides of the sheet, have an additional requirement principally that the surface of this impression cylinder must not smear the as yet undried ink on the surface of the printed picture on the sheet of paper which is transferred to the impression cylinder of the second printing unit, as well as accumulate ink on the surface of this impression cylinder. One of the known types of impression cylinders uses for this purpose a spanned net anti-corrosive material arranged on the surface of the impression cylinder which divides the surface of the cylinder into a number of equally spaced points. The contact surface of the cylinder is considerably reduced by means of this spanned net. The transferred paper sheet is contacted by this net and thereby the danger of smearing of the print on the sheet is reduced. This type of impression cylinder has the disadvantage of requiring a laborious cleaning of the net because as the net is used for a time, it becomes contaminated with ink. The ink also accumulates under the net and thereby increases the dimension of the cylinder which results in smudging on the area of the printed sheet.
There is also known an impression cylinder wherein the aforementioned net is replaced by a ragged metal foil which also is spanned on the surface of the cylinder. This foil usually is made from aluminum or a chromium-plated aluminum foil. An advantage of the foil in comparison with the spanned net resides in that the ink has no possibility of accumulating under the foil and additionally the cleaning of this foil is easier to accomplish than that of a metal net.
The disadvantage of the use of a ragged foil resides in that the foil becomes worn or damaged in a relatively short period of time, particularly along the edges of the cylinder over which the foil is spanned. The exchanging of the foil is difficult and time-consuming as well. A further disadvantage resides in the necessity of small tolerances for the thickness of the foil because the latter has an influence on the dimension and accurate geometry of the cylinder surface.
There is also known a cylinder, the surface of which is provided with a layer of chromium and has a special surface roughness. The disadvantage of this type of cylinder resides in that the layer of chromium is very thin and is only about 7 to 12 millimicrons thick. A thicker layer of chromium results in an inaccurate surface geometry for the cylinder due to accumulations of chromium along the edges of the cylinder. This accumulation is generally larger than is permissible for an acceptable accuracy. An additional working of the cylinder surface is not possible because the already existing surface roughness would then be removed. This roughness of the operative surface of the cylinder is essential for proper functioning of the cylinder. A thin layer of chromium is generally porous and this porosity generally reduces the corrosion-resistance properties considerably.